In the field of micro-optics, refractive elements such as micro-lenses are difficult to fabricate using photolithographic techniques. Photolithography has been developed primarily for fabricating binary structures in photo-resist; that is, structures with substantially two distinctive heights or levels: in the case of a positive photo-resist, this results in a lower level where the photo-resist was removed, and an upper level where the remaining photo-resist material covers the substrate. The binary type of resist profile has been researched and optimized extensively for the last twenty years in order to reach the highest possible level of resolution, but such a conventional photolithographic process is unsuitable for the continuous relief profiles required for refractive micro-optics elements. In the micromachining of micro-optics, a special technique is required to create continuous resist profiles for analog micro-optical elements such as micro-lenses and prisms.
Fabrication techniques recently developed for large analog resist profiles in thick photo-resist include gray-scale, half-tone, and binary phase mask technology. But current techniques available for fabricating analog micro-optics are both costly and highly specific to each element and application.
Various prior art patents have been proposed in this field. See for example, U.S. Pat. Nos. 5,482,800; and 6,613,498. U.S. Pat. No. 5,285,517 describes high energy beam sensitive glasses. U.S. Pat. No. 6,562,523 describes a direct write all-glass photomask blanks technique. U.S. Pat. Nos. 6,071,652; 6,420,073; 6,524,756, and 6,638,667 deal with the gray-level mask. None of these patents overcomes the problems with the prior art.
The gray-scale mask provides analog optical transmittance with continuously varying optical density. Gray-scale masks utilize the properties of a high-energy beam sensitive (HEBS) glass plate in order to form continuous relief profiles in the photo-resist. Gray-scale masks have two main drawbacks: high cost and strict dependence on the optical density of the photo-resist being used. It is necessary to characterize the thickness of the resist in terms of the optical density for a specific exposure tool in order to design a proper optical density map on a gray-scale mask.
Half-tone masks create analog optical transmittance by use of a square dot array representing continuous optical density. By varying the pixel density or size, half-tone masks are capable of creating analog optical transmittance for the incident exposure light. However, this technique suffers from the pixel aperture diffraction effect, and also requires the adjustment of pixel density for a specific exposure tool. Half tone techniques have various other problems such as more light being diffracted resulting in more noise in the image plane. Furthermore, the depth of focus is limited.
The concept of utilizing alternating phase shift on the photo-mask first came up in the late 1980s as a technique of enhancing resolution by overcoming the diffraction limit of the imaging system. The pure phase mask does not cause the light to scatter due to the edge diffraction as half-tone masking does. The phase shift mask has become a mature, standard technique for resolution enhancement in the semiconductor industry, and is mainly used for dense periodic patterns of sub-micron resolution. The phase shift is usually implemented in the alternating opening region of a binary amplitude chrome mask by etching the mask substrate for a phase shift of half wavelength. A pure binary phase mask is sometimes used for fabricating high frequency sinusoidal gratings in the photo-resist with half period of the mask. However, the phase grating mask technique has not been used for creating analog resist profiles as have gray-scale and half-tone mask techniques. This technique is restricted to resolution enhancement for binary patterns and is not suited for the fabrication of analog resist profiles.
Thus, there exists a need for a method to fabricate large analog resist profiles in a variety of both thin and thick photo-active polymers for the fabrication of refractive micro-elements using projection systems at a variety of wavelengths that solves the problems of the prior art.